Cold Heavy Oil Production with Sand (CHOPS) is defined as primary heavy oil production that involves the deliberate initiation of sand influx into a perforated oil well, and the continued production of substantial quantities of sand along with the oil, perhaps for many years. CHOPS is a non-thermal primary method using high pressure drops in the formation, such as a sedimentary rock bed. Sand is produced along with heavy oil. It is feasible to achieve oil rates of 5-20 m3 per day. Around 15-20 percent of original oil in place can be extracted. The produced fluid may contain 1-8 percent sand. Average well life may be 5-8 years. It is typical to have high initial oil rate followed by a gradual decline. CHOPS well operations are feasible at low enough pressure to allow continuing sand production.
Wormholes, or high porosity, high permeability channels, tend to develop and grow in the weakest sand and toward highest pressure gradient. Wormholes may not grow from each perforation of the oil well; however they tend to be stable when they do develop. For many operators of CHOPS, oil wells are drilled based on evaluation of porosity and resistivity log measurements of reservoirs, which are subsurface rock bodies having sufficient porosity and permeability to store and transmit oil. The drilled wells usually contain apparent pay sections of sufficient cumulative pay thickness to Justify casing and completion. However, what is not so apparent is how productive those pay sections may be in production. As used herein, the term “pay” refers to a reservoir or portion of a reservoir that contains economically producible oil contents, and the term “completion” refers to configuring a production casing string set across the reservoir interval and perforated to allow communication between the formation and wellbore.
Conventional practice is to select sand with the highest porosity and resistivity along the wellbore, then perforate these areas and attempt to produce from these sands. This method has shown results with a less than 50 percent success rate. As an example, FIG. 1 shows log measurements of two oil wells in the vicinity of each other. The log measurements are identified by the oil well number 1-34-XX-XX and 16-27-XX-XX. The target depth for wellbore perforation is marked in locations 101, 102, 103, and 104, which has been identified by a conventional criteria of porosity greater than 24 percent and resistivity greater than 10 ohm.m. The pay thickness of the oil well 1-34-XX-XX has been determined to be 4 meters while that of the oil well 16-27-XX-XX was determined to be 6 meters. However, the cumulative production outputs of these two wells, at 654 m3 and 17500 m3 respectively, clearly do not correlate with the apparent pay thickness.
FIG. 2 shows a specimen cross section of a sanding experiment and related characteristics. The sanding experiment predicts that sand failure, shown as a slot-like failure 201 in FIG. 2, will propagate in the minimum stress direction 202. This experiment, as well as other similar experiments and researches in related fields relate to sands, in general, and do not make references to wormholes in heavy oil production or its application in the field. Specifically, in the prior art, sand production has been viewed as a common and very damaging problem in hydrocarbon production from elastic reservoir beds. On the other hand, CHOPS has been viewed as a low cost operation and therefore research efforts have been limited.
Related geomechanical research has been published by Bezalel Haimson et al., “Borehole Breakouts in Berea Sandstone: Two Porosity-Dependent Distinct Shapes and mechanisms of Formation” SPE/ISRM 47249, SPE/ISRM Eurock 1996, Trondheim, Norway, 8-10 Jul. 1996 and Julian Heiland et al., “Influence of Rock Failure Characteristics on Sanding Behavior: Analysis of Reservoir Sandstones from the Norwegian Sea” SPE 98315, 2006 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, L. A., 15-17 Feb. 2006.